Tailgate carts have become very popular with gardeners and landscapers in recent times. These carts are hitched to a truck, and provide a large cargo area for the storage and transportation of mowers, ladders, tillers, edging tools, weed-whackers, and other gardening and landscaping equipment. These tailgate carts have a low center of gravity, making them very stable in transporting heavy gardening tools. A tailgate cart of the kind that is described above, is shown and described in the parent invention, U.S. Pat. No. 7,029,050.
One outstanding feature of these tailgate carts is their large access door. The door is extends many feet above the Loading-bed, and its bottom is pivotally connected to the base of the tailgate frame surrounding the loading bed. To access the equipment disposed in the bed of the cart, the door is pulled down about its pivots, until the top of the door touches the roadway. In this position, the door acts like a loading and unloading ramp. The equipment is then easily rolled off the cart for the next landscaping job.
The door of the cart is very heavy, and is latticed to reduce its weight for handling purposes. Yet, despite being latticed, the tailgate door is not easily lifted, or lowered.
In the aforementioned parent, patented invention, a double spring assist device for opening and closing the door, is illustrated and described. This double spring device is mounted on the top of the frame of the tailgate bed. The springs are free to extend along the frame in tandem. A cable or chain is attached to the first one of the springs via a double pulley, and as the door is lowered and raised, the springs are caused to extend and contract, accordingly. The extension of the first and second springs counterbalance the force of the pivoting door in accordance with Hook's law: F1+F2=K1X12+K2X22, where F1+F2 are the forces required to lower and lift the door, K1 and K2 are the respective spring constants in pounds/foot2; and X1 and X2 are the distances traveled by each spring in feet.
One of the major problems with the parent assist device is that the springs tend to pull on each other as they react to the lowering of the door, and do not uniformly apply a matching force response. It was found that by making the second spring a compression spring, and the first spring a tension spring, a more uniform force response was achieved.
Another problem that was uncovered was the operation of the pulleys in the forward perch unit. These twin pulleys tended to guide the cable in a vertical fashion. This caused excessive cable wear over time. It was found that it was much better to allow the cable to swing freely without being guided, as it departs the spring assembly. To achieve this, the forward cable is now fed through a ball race consisting of five rotating balls in juxtaposition to each other. The cable riding over these rotating balls does not experience any vertical guiding forces, which tend to wear the cable over time, as aforementioned. The result is that the forces exerted upon the system during the lowering of the door, is now more uniform, and the cable assist device is more rugged.
The present invention, seeks to provide a more uniform counterbalancing of the spring force vis-à-vis the lateral force component of the gravity vector of the tailgate door. This result is accomplished by different counter-balancing, i.e. two springs, one of which is in tension, and the other of which is in compression. The two counterbalancing springs act more smoothly in dealing with the forces produced by gravitational pull of the tailgate door. The invention has two springs of unequal length disposed in seriatim. The smaller, or first, tension spring shoulders most of the load, when the tailgate door is being initially lowered. As the door reaches a 45° angle with respect to the tailgate base, the second, longer compression spring begins to shoulder the existing force. As the top of the door reaches the floor, both the first and second springs are fully operative to allow the door to be gently placed upon the ground. In the equation, Force=F1+F2=K1X12+K2X22, the extension of the springs of different length and/or different spring constant of this invention, furnish a non-linear response to the non-linear lateral force component of the door. The first spring supplies a lower counterbalancing force, than the second spring. The counterbalancing force pattern is less in the beginning of the door travel, but greater at the end of the door travel. In other words, the nonlinear response pattern is molded to more closely fit the force vectors of the tailgate door.